The number of components that can be integrated onto a semiconductor chip has been increasing rapidly. At the same time, the number of different types of integrable components has also increased. Today's integrated circuits (IC's) may have analog, digital, or mixed signal components or some combination of these types.
Testing analog and mixed signal IC's poses problems different from those encountered in testing digital integrated circuits. Analog and mixed signal IC's deal with analog signals, and the transistors as well as other components may or may not operate in a linear manner.
Analog circuit testing is generally "functional" in that it attempts to associate defects in the circuit with failure of the circuit to perform its intended function. One approach to functional testing is to conceptualize the circuit as a set of functional modules. This reduces the complexity of the test operation as compared to testing each primitive component of the circuit.
When the circuit is not integrated, block functional testing is not especially difficult. Each block is tested separately by applying input signal values and measuring outputs.
For IC's, functional testing is easily implemented in "end to end" testing of the entire circuit, because the circuit inputs and outputs are available for applying input signals and measuring outputs. However, for block level testing, where a block has one or more inputs or outputs that are not the same as the circuit inputs or outputs, test values cannot be physically applied to the block inputs and block outputs cannot be physically measured.
One solution to block functional testing is to manually propagate block inputs and outputs to circuit inputs and outputs. In the past, this has been done manually, but for complex circuit that computational burden is large. Automated techniques for backward and forward propagation have also been described. U.S. patent Ser. No. 970,973, now abandoned entitled "A Method for Generating Analog Test Plans", assigned to Texas Instruments, Incorporated, describes a method for testing analog integrated circuits, using forward and backward path propagation. Test values to be applied to block inputs and expected values at block outputs are attempted to be propagated to the circuit inputs and outputs.
Existing test plan generation methods fail to provide tests under some circumstances. For example, they do not provide tests that require current setting, ac analysis, or transient analysis. They do not provide tests for circuits with feedback. A need exists for a method for generating all types of tests for all types of analog circuits.